Counter-ion specific effects on charge and solvent trapping in poly(vinylferrocene) films

This study determined the effect on the electrochemical quartz crystal microbalance's (EQCMs) response of substituting tetrafluoroborate and hexafluorophosphate as counter ions into a poly(vinylferrocene) (PVF) film that had been prepared initially with perchlorate ion as the counter ion in methylene chloride. This PVF+ClO4− film was redox cycled successively in aqueous 0.1 M sodium perchlorate, 0.1 M sodium hexafluorophosphate and 0.1 M sodium tetrafluoroborate solutions. Cyclic voltammetric EQCM experiments determined the changes in film mass and charge during the first and subsequent potential cycles. In all bathing solutions, the total mass change and charge consumed during the first oxidation cycle were larger than in subsequent potential cycles; thereafter there were negligible changes. This is rationalized by the trapping of one or more oxidized forms of PVF produced in the first oxidation cycle, and two possible detailed mechanisms are given. The extent of film oxidation was controlled kinetically, and it changed in the counter ion order tetrafluoroborate>perchlorate>hexafluorophosphate. Less film oxidation occurred at high potential scan rates for all counter ions. At the highest potential scan rate (0.080 V s−1) the amount of water entering the oxidized film from solution was greater in tetrafluoroborate, than in perchlorate, or than in hexafluorophosphate solutions. At slower potential scan rates these differences still existed but were less obvious. Free volume restraints within the film limited the total amount of counter ion and solvent that could be introduced under kinetically controlled conditions.